i18n(ja): create reference/content-loader-reference.mdx

[nemuvski]

Description

This PR adds a Japanese translation to reference/content-loader-reference.mdx.

The original English version: https://docs.astro.build/en/reference/content-loader-reference/ (on github)

I found the Content Loader API to be very useful, so I decided to commit a Japanese translated version. 🙌

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Warnings reference

Icon	Description
🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

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